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Automatic Direction Finder

A. DESCRIPTION

One of the older types of radio navigation is the automatic direction finder (ADF) or non-directional
beacon (NDB). The ADF receiver, a "backup" system for the VHF equipment,
can be used when line-of-sight transmission becomes unreliable or when there is no VOR
equipment on the ground or in the aircraft. It is used as a means of identifying
positions, receiving low and medium frequency voice communications, homing, tracking, and
for navigation on instrument approach procedures.

The low/medium frequency navigation stations used by ADF include non-directional
beacons, ILS radio beacon locators, and commercial broadcast stations. Because commercial
broadcast stations normally are not used in navigation, this section will deal only with
the non-directional beacon and ILS radio beacon.

A non-directional radio beacon (NDB) is classed according to its power output
and usage:

the L radio beacon has a power of less than 50 watts (W),

the M classification of radio, beacon has a power of 50 watts up to 2,000 W;

the H radio beacon has a power output of 2,000 W or more;

the ILS radio beacon is a beacon which is placed at the same position as the outer
marker of an ILS system (or replaces the OM).

B. LIMITATIONS AND BENEFITS

Pilots using ADF should be aware of the following limitations:

Radio waves reflected by the ionosphere return to the earth 30 to 60 miles
from the station and may cause the ADF pointer to fluctuate. The twilight effect is most
pronounced during the period just before and after sunrise/sunset. Generally, the greater
the distance from the station the greater the effect. The effect can be minimized by
averaging the fluctuation, by flying at a higher altitude, or by selecting a station with
a lower frequency (NDB transmissions on frequencies lower than 350 kHz have very little
twilight effect).

Mountains or cliffs can reflect radio waves, producing a terrain effect.
Furthermore, some of these slopes may have magnetic deposits that cause indefinite
indications. Pilots flying near mountains should use only strong stations that give
definite directional indications, and should not use stations obstructed by mountains.

Shorelines can refract or bend low frequency radio waves as they pass from
land to water. Pilots flying over water should not use an NDB signal that crosses over the
shoreline to the aircraft at an angle less than 30°. The shoreline has little or no
effect on radio waves reaching the aircraft at angles greater than 30°.

When an electrical storm is nearby, the ADF needle points to the source of
lightning rather than to the selected station because the lighting sends out radio waves.
The pilot should note the flashes and not use the indications caused by them.

The ADF is subject to errors when the aircraft is banked. Bank error is
present in all turns because the loop antenna which rotates to sense the direction of the
incoming signal is mounted so that its axis is parallel to the normal axis of the
aircraft. Bank error is a significant factor during NDB approaches.

While the ADF has drawbacks in special situations, the system does have some general
advantages. Two of these benefits are the low cost of installation of NDBs and their
relatively low degree of maintenance. Because of this, NDBs provide homing and
navigational facilities in terminal areas and en route navigation on low-level airways and
air routes without VOR coverage. Through the installation of NDBs many smaller airports
are able to provide an instrument approach that otherwise would not be economically
feasible.

The NDBs transmit in the frequency band of 200 to 415 kHz. The signal is not
transmitted in a line of sight as VHF or UHF, but rather follows the curvature of the
earth; this permits reception at low altitudes over great distances.

The ADF is used for primary navigation over long distances in remote areas of Canada.

C. ADF COMPONENTS

The NDB Control Panels figure, on the right, shows the major ADF
components except the receiving antenna, which on most light aircraft is a length of wire
running from an insulator on the cabin to the vertical stabilizer.

l . RECEIVER: Controls on the ADF receiver permit the pilot to tune the station desired
and to select the mode of operation. \When tuning the receiver the pilot must positively
identify the station. The low or medium frequency radio beacons transmit a signal with
1,020 Hertz (cycles per second [Hz]) modification keyed to provide continuous
identification except during voice communications. All air facilities radio beacons
transmit a continuous two- or three-unit identification in Morse code, except for ILS
front course radio beacons which normally transmit a continuous one letter identifier in
Morse code. The signal is received, amplified, and converted to audible voice or Morse
code transmission. The signal also powers the bearing indicator.

Tuning the ADF- To tune the ADF receiver, the pilot should follow these steps:

turn the function knob to the RECEIVE mode. This turns the set on and selects the mode
that provides the best reception. Use the RECEIVE mode for tuning the ADF and for
continuous listening when the ADF function is not required;

select the desired frequency band and adjust the volume until background noise is heard;

with the tuning controls, tune the desired frequency and then re-adjust volume for best
listening level and identify the station;

to operate the radio as an automatic direction finder, switch the function knob to ADF;
and

the pointer on the bearing indicator shows the bearing to the station in relation to the
nose of the aircraft. A loop switch aids in checking the indicator for proper operation.
Close the switch. The pointer should move away from the bearing of the selected station.
Then release the switch; the pointer should return promptly to the bearing of the selected
station. A sluggish return or no return indicates malfunctioning of the equipment or a
signal too weak to use.

2. CONTROL BOX - DIGITAL READOUT TYPE: Most modern aircraft have this type of control
in the cockpit. In this equipment the frequency tuned is displayed as a digital readout of
numbers rather than tuning a frequency band.

ADF - Automatically determines bearing to selected station and displays it on the RMI.
Uses sense and loop antennae.

ANT - Reception of Radio signals using the sense antenna. Recommended for tuning.

TEST - Performs ADF system self-test. RMI needle moves to 315°.

b) Frequency Selector Switches. Three concentric knobs, permit selection of
operating frequency. Two frequencies can be preselected. Only one can be used at a time.
The transfer switch indicates the frequency in use.

c) Selected Frequency Indicators. Provides a visual read-out of the
frequencies selected. The numbers can be printed on drums that rotate vertically or, in
more modern sets, they arc displayed by light emitting diodes.

3. ANTENNAE: The ADF receives signals on both loop and sense antennae. The loop antenna
in common use today is a small flat antenna without moving parts. Within the antenna are
several coils spaced at various angles. The loop antenna senses the direction of the
station by the strength of the signal on each coil but cannot determine whether the
bearing is TO or FROM the station. The sense antenna provides this latter information, and
also voice reception when the ADF function is not required.

4. BEARING INDICATOR: As mentioned above, the bearing indicator (see
Fixed Card Bearing Indicator figure, on the right) displays the bearing to the
station relative to the nose of the aircraft. If the pilot is flying directly to the
station, the bearing indicator points to 0°. An ADF with a fixed card bearing indicator
always represents the nose of the aircraft as 0° and the tail as 180°.

Relative bearing (see NDB Bearings figure,
on the left) is the angle formed by the intersection of a line drawn through the
centerline of the aircraft and a line drawn from the aircraft to the radio station. This
angle is always measured clockwise from the nose of the aircraft and is indicated directly
by the pointer on the bearing indicator.

Magnetic bearing (see NDB Bearings figure, on the left) is
the angle formed by the intersection of a line drawn from the aircraft to the radio
station and a line drawn from the aircraft to magnetic north. The pilot calculates the
magnetic bearing by adding the relative bearing shown on the indicator to the magnetic
heading of the aircraft. For example, if the magnetic heading of the aircraft is 40° and
the relative bearing 210°, the magnetic bearing to the station is 250°. Reciprocal
bearing is the opposite of the magnetic bearing, obtained by adding or subtracting 180°
from the magnetic bearing. The pilot calculates it when tracking outbound and when
plotting fixes.

D. ADF OPERATIONS

1. MONITORING: Since the ADF receiver normally has no system failure or "OFF
warning flags to provide the pilot with immediate indication of a beacon failure or
receiver failure, the ADF audio must be monitored. The "idents" should be
monitored anytime the ADF is used as a sole means of en route navigation. During the
critical phases of approach, missed approach and holding, at least one pilot or flight
crew member shall aurally monitor the beacon "idents" unless the aircraft
instruments automatically advise the pilots of ADF or receiver failure.

2. HOMING: One of the most common ADF uses is "homing to a station".
When using this procedure, the pilot flies to a station by keeping the bearing indicator
needle on 0° when using a fixed-card ADF (See Homing to an NDB figure,
below right). The pilot should follow these steps:

tune the desired frequency and identify the station. Set the function selector knob to
ADF and note the relative bearing;

turn the aircraft toward the relative bearing until the bearing indicator pointer is
0°; and

continue flight to the station by maintaining a relative bearing of 0°.

The figure Homing to an NDB, on the right, shows that if the pilot
must change the magnetic heading to bold the aircraft on 0° the aircraft is drifting due
to a crosswind. If the pilot does not make crosswind corrections, the aircraft flies a
curved path to the station while the bearing indicator pointer remains at zero. The
aircraft in position 2 must keep changing its heading to maintain the 0°
relative bearing while flying to the station.

The bracketing method used here is basically the same as that explained elsewhere. The
major difference is that bracketing a VOR requires the pilot to bracket a radial
identified by the TB needle, whereas bracketing an ADF magnetic bearing requires the pilot
to identify it by using both the bearing indicator and the heading indicator.

Assume the pilot of the aircraft in position l (see
Bracketing an NDB Magnetic Bearing figure, on the right) desires to intercept the
090° magnetic bearing to the non-directional beacon. The pilot then sets up an intercept
angle of 30° which is shown by the 120° heading of the aircraft. The ADF pointer
indicates 340°. Because the magnetic bearing equals the magnetic heading of the aircraft
and the relative bearing, the pilot adds 120° (the relative bearing) and finds that the
aircraft is on the 100° magnetic bearing.

NOTE:Whenever the aircraft heading and relative bearing equal more than 360° the pilot
should subtract 360° from the resulting figure. The pilot then follows the rest of the
bracketing procedure.

3. TRACKING FROM A STATION: A pilot can use ADF to track from a station by employing
the principles of bracketing a magnetic bearing. The Tracking from an NDB
figure, below left, illustrates an aircraft tracking outbound from a station with a
crosswind from the north. The reciprocal bearing is 090°, and the pilot tracks this
bearing by flying the aircraft with 10° of wind correction. The pilot knows that the
aircraft is tracking a reciprocal bearing because the heading indicator (080°) and
relative bearing (190°) equal the magnetic bearing (270°).

4. POSITION FIX BY ADF: The ADF receiver can help the pilot to make a definite position
fix by using two or more stations and the process of triangulation. To determine the exact
location of the aircraft, the pilot should use these procedures:

locate two stations in the vicinity of the aircraft. Tune and identify each;

set the function selector knob to ADF, then note the magnetic heading of the aircraft as
read on the heading of the aircraft as read on the heading indicator. Continue to fly this
heading and tune in the stations previously identified, recording the relative bearing for
each station;

add the relative bearing of each station to the magnetic heading to obtain the magnetic
bearing. Correct the magnetic bearing for east-west variation to obtain the true bearing;
and

plot the reciprocal for each true bearing on the chart. The aircraft is located at the
intersection of the bearing lines (see Position Fix by NDB figure, on the
right).

5. TIME COMPUTATION TO FLY TO A STATION: Computing time to the station is basically the
same for ADF as it is for VOR (refer to Article 2.2.3. E (2)) therefore, a brief example
is sufficient here. The basic procedure is to:

turn the aircraft until the ADF pointer is either at 090°s or 270°s and note the time;
and

fly a constant magnetic heading until the ADF pointer indicates a bearing change of
10°. Note the time again and apply the following formula:

(TIME IN SECONDS BETWEEN BEARING CHANGE)/(DEGREES OF BEARING CHANGE) equals TIME TO
STATION IN MINUTES.

For example, if it takes 45 seconds to fly a bearing change of 10°, the aircraft is:

45 / 10 = 4.5 min from the station.

To find distance to a station multiply time by distance covered in one minute using TAS
or preferably G/S.

As with VOR procedures, a 10° bearing change is the simplest and easiest to use in
making this calculation. If the pointer moves so rapidly that a satisfactory time check
cannot be obtained during a 10° bearing change, this rapid movement indicates that the
aircraft is very close to the station.

The material for this section is reproduced from
the publication, FROM THE GROUND UP, with the permission of its copyright owner,
Aviation Publishers Co. Ltd. No further reproduction is authorized, in any
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